MOVING PICTURES 



449 



the film. The length of these films may vary from three 

 hundred to five hundred feet. 



When an object is to be taken which is moving both hori- 

 zontally and vertically, like a flying machine, a special kind 

 of tripod is used. This has two sets of gears and wheels, 

 one of which moves the camera in a horizontal plane, while 

 the other moves it up and down. 

 This requires two men to operate 

 it, one to turn the handle for tak- 

 ing the pictures, and the other to 

 move the camera so that the ob- 

 ject to be photographed is kept in 

 the center of the field of vision. 



Preparing the film. Special 

 means have to be employed for 

 handling long films in develop- 

 ment. The films are wound 

 around long wooden frames or 

 reels. The principle involved is 

 the same as that described in 

 Chapter VII. The films for one 

 subject are often developed in 

 several parts and then are 

 fastened together by means of 

 a transparent cement. There is 

 often great waste in the film, 

 sometimes as much as twenty per 

 cent being thrown away on ac- 

 count of imperfections. 



In order to make a positive that can be used in the pro- 

 jecting apparatus another strip of perforated film similar to 

 the first one is used, except that it is less sensitive to light. 

 The principle is the same as that involved in making ordi- 

 nary prints, only different devices must be used on account 

 of the great length of film. 



2G 



FIG. 178. A motion-picture 

 printing machine. 



A- A' Rollers for negative film; 

 B-B' Rollers for positive film; 

 C Film gate where positive is 



held over negative for printing ; 

 E Unexposed positive film ; 

 E' Exposed or printed positive 



film; 

 F Light which, shining through 



film gate, imprints image of 



negative on positive. 



